Rat ventral prostate chromatin contains protein phosphokinase activity which is sensitive to the androgenic status of the animal. This activity, due to the presence of several endogenous protein phosphokinases, is apparent towards exogenous substrates such as dephosphophosvitin and lysine-rich histone. Orchiectomy of adult rats results in a rapid decline in the chromatin-associated protein phosphokinase (assayed with dephosphophosvitin as substrate) so that a 30% decline in activity was observed as early as 9 hr post-orchiectomy; The activity was about 50% of the control values at 18 hr and had reduced to about 20% of the controls by 48 hr post-orchiectomy. These changes were prevented by testosterone treatment of orchiectomized animals. Further, the decline in chromatin-associated protein kinases appeared to precede any gross changes in the chemical composition of chromatin. Unlike the cytosol histone kinase of rat ventral prostate, the chromatin-associated activity is sensitive to the androgenic status of the animal; it showed a 43% decline at 48 hr post-orchiectomy. Studies are underway to examine the time course of the effect of a single injection of testosterone into castrated animals, with respect to the modulation of the various chromatin-associated protein phosphokinases.